1. Field of the Invention
The present disclosure generally relates to wireless communications and more particularly relates to systems and methods for retrieving buffered data from an access point.
2. Background Information
Among other things, FIG. 1 illustrates a typical network configuration for communicating data between stations via an access point in a wireless local area network (WLAN) or 802.11-based network. As illustrated in the non-limiting example of FIG. 1, a network 140 may be coupled to access point 130. In some embodiments, the network 140 may be the Internet, for example. Access point 130 can be configured to provide wireless communications to various wireless devices or stations 110, 120, 124. Depending on the particular configuration, the stations 110, 120, 124 may be a personal computer (PC), a laptop computer, a mobile phone, a personal digital assistant (PDA), and/or other device configured for wirelessly sending and/or receiving data. Furthermore, access point 130 may be configured to provide a variety of wireless communications services, including but not limited to: Wireless Fidelity (WIFI) services, Worldwide Interoperability for Microwave Access (WiMAX) services, and wireless session initiation protocol (SIP) services. Furthermore, the stations 110, 120, 124 may be configured for WIFI communications (including, but not limited to 802.11, 802.11b, 802.11a/b, 802.11g, and/or 802.11n).
Access point 130 periodically broadcasts a beacon frame to various stations at a beacon period. The beacon frame is used by an access point to announce its presence and to relay information. For example, if station 110 is a laptop and is powered up or is transported to a location within range of access point 130, station 110 listens for a beacon frame from all access points in its range. Each access point within range transmits a beacon frame and depending on the system, the user at station 110 can select which access point to use, thereby making an association between station 110 and the access point.
In order to save power, stations can be put into standby mode. While sleeping, the access point buffers data intended for the station. Also, for the purposes of this disclosure the term “sleep mode” will be taken to mean an operating state entered by a computing device either upon initiation by a user or after expiration of a period of sufficient inactivity in which the amount of power supplied to the device is reduced as compared to the amount supplied during normal operation. The stations in standby mode wake up to receive the beacon frame. Contained within the beacon frame is a traffic indication map (TIM) element which indicates for which stations the access point has buffered data waiting. If the station has determined that the access point has buffered data for it, the station can retrieve the data by sending a Power Saving-POLL (PS-Poll) frame.
FIG. 2 shows an example of the retrieval of buffered data by the station using a PS-Poll frame. In sequence 200, PS-Poll frame 202 sent by the station is followed immediately after a short interframe space (SIFS) by the data frame 206 sent by the access point. After another SIFS, the station responds with acknowledgement (ACK) 210.
As illustrated in FIG. 3, the stations in standby mode wake up at the target beacon transmission time (TBTT) in order to receive the beacon frame. After a beacon frame is received, the station can determine the TBTT for the next beacon frame from the start time of the current beacon frame and the beacon interval value transmitted in the current beacon frame. In the timeline shown here, beacon frames 302, 304, 306, 308, and 310 represent five beacon frames transmitted by the access point. The frequency of the beacon frames is represented by the period equal to the beacon interval. Each of the beacon frames in the example contains a TIM element; specifically, beacon frames 302, 304, 306, and 310 contain TIM elements 312, 314, 316, 318 and 320, respectively. Periodically, the TIM element in the beacon frame is a delivery traffic indication map (DTIM), which indicates after the beacon frame the access point will transmit buffered multicast or broadcast data.
Stations in standby mode, in addition to determining whether the access point has any buffered data waiting for it, may also require additional information from the beacon frame. For example, the access point may announce a channel switch through a channel switch announcement information element which can be included in a beacon frame or some other announcement mechanism such as a probe response as described below. The channel switch announcement information element indicates that the Basic Service Set (BSS) will move to another channel shortly. This information is needed by the station so that it can follow the channel change; otherwise, it will wake only to find that the beacon frame no longer is transmitting on the present channel.
In order for any station in standby mode, which has been associated with an access point to determine whether that access point has buffered data waiting for that station, the station must periodically wake up to receive the TIM element within the beacon frame. However, the length of beacon frames has grown over time with the progression of standards and implementation of more and more features. This would require the station to stay awake longer to receive a lengthy beacon frame, which can cause a station to consume more power in standby mode. Furthermore, beacon frames are generally transmitted at a low physical layer (PHY) rate, often at the lowest PHY rate allowable. Because the transmission rate is so low, the station must stay awake longer to receive the beacon frame. This has an adverse effect on the battery life of handheld devices. Accordingly, various needs exist in the industry to address the aforementioned deficiencies and inadequacies.